Hydraulic drive systems can be employed to provide mechanical power to drive machinery such as a positive displacement pump with a reciprocating piston, and other machinery that uses hydraulic fluid pressure to drive mechanical movements. In such hydraulic drive systems, hydraulic fluid pressure can be measured to provide an indicator of an operational condition, and such indicators can be used to control the hydraulic drive system. For example, co-owned Canadian patent no. 2,527,122, entitled, “Apparatus and Method for Pumping a Fluid From a Storage Vessel and Detecting When the Storage Vessel is Empty” (the '122 patent) discloses an apparatus comprising a hydraulically driven reciprocating piston pump that pumps a process fluid from a storage tank and a method comprising measuring hydraulic fluid pressure to determine when the storage tank is empty. For process fluids such as cryogenic fluids, commercially practical level sensors are not yet available, so such a method of determining when the storage tank is empty and preventing the pump from operating when the storage tank is empty is useful. The method taught by the '122 patent comprises measuring peak hydraulic system pressure and determining that the storage tank is empty when peak hydraulic system pressure falls below a predetermined threshold value for a predetermined number of times, indicating that the pump is encountering less process fluid resistance during the pumping stroke. When the storage tank is determined to be empty, the electronic controller for the hydraulic system can be programmed to switch to pumping process fluid from a different storage tank. While this method works, a challenge associated with this approach is that peak hydraulic system pressure can change responsive to factors other than the amount of process fluid being pumped. For example, peak hydraulic system pressure can also change responsive to the pressure of the process fluid in the system to which it is being pumped, since downstream process fluid pressure correlates to the resistance against the pump piston during a discharge stroke. Resistance to pump piston movement can also be a function of kinetic friction, whereby changes in hydraulic fluid flow rate, caused by changes in the speed of a hydraulic pump that delivers the hydraulic fluid to the hydraulic drive, can also influence peak hydraulic system pressure in the hydraulic drive. Accordingly, the method taught by the '122 patent, which relies upon a measurement of peak hydraulic system pressure, can be improved if the expected peak hydraulic system pressure is adjusted to account for other factors that affect the peak hydraulic system pressure, such as process fluid pressure and hydraulic drive speed. The utility of this method is not confined to hydraulically driven pumps. For different hydraulically driven apparatuses, such as, for example, a hydraulic press or an extruder, if peak hydraulic system pressure is less than expected, this can be an indication that there is a smaller than expected quantity of the material that is being worked on, indicating that the supplied material needs to be replenished or that it is time to stop the machinery; here too, peak hydraulic pressure can be variable as a function of normal operating variables such as hydraulic pump speed or hydraulic fluid flow rate.
Referring still to the example of a hydraulically driven reciprocating pump, the efficiency of the pump can be improved by preventing the pump piston from short stroking, which occurs if the pump piston does not extend or retract fully, resulting in an incomplete piston stroke. This is a problem for both single-acting and double-acting piston pumps, because a short stroke prevents the pump piston chamber from being fully charged with process fluid and/or from fully discharging the process fluid. Conventional hydraulic drives can use magnetic proximity sensors to detect when the piston has reached the end of a piston stroke, but this approach adds to the cost and maintenance required since two sensors are required for each hydraulic drive piston. Another approach is to use a flow meter to measure the hydraulic fluid flow and calculate when the hydraulic piston has reached the end of its stroke based on the known volume of the hydraulic cylinder. However, with this approach, the flow meter can be expensive and inaccuracies can be introduced by other factors, such as the accuracy of the flow meter or if hydraulic fluid leakage in the system. Co-owned Canadian patent application 2,476,032, entitled, “Hydraulic Drive System and Method of Operating a Hydraulic Drive System” (the '032 application) discloses a method of preventing short stroking by using a shuttle valve disposed in the hydraulic piston that allows hydraulic fluid to flow through the piston at the end of each piston stroke. This allows the hydraulic piston to complete each stroke without being driven into and damaging the end plates, which permits a controller to be programmed to estimate when the piston has reached the end of a stroke based on at least one of hydraulic pump speed, hydraulic fluid pressure, or elapsed time. The operation of the shuttle valve allows the controller some leeway to ensure that the hydraulic piston stroke is completed before it sends an electronic signal to a flow switching device to switch hydraulic fluid flow direction and the direction of hydraulic piston movement.
FIG. 1 is a graph that plots hydraulic system pressure and pumping state against time for a hydraulic drive system with the hydraulic fluid pump operated with a constant speed. The hydraulic system pressure plotted by line 101 is measured by a sensor associated with a conduit that connects the discharge outlet from a hydraulic pump to a hydraulic drive unit. In this example, the hydraulic drive unit comprises a reciprocating hydraulic piston and the pumping state plotted by line 105 shows whether a piston in the hydraulic drive unit is extending or retracting. A value of 1 for the pumping state indicates that the hydraulic piston is extending and doing work as shown by the correlation with the peak hydraulic system pressure. A value of 2 for the pumping state indicates that the hydraulic piston is retracting, and in this example the pump driven by the hydraulic drive unit is a single-acting pump so the hydraulic system pressure during the retracting stroke is much lower. The plotted data relates to a hydraulic drive unit that is driving a single-acting positive displacement piston pump, that pumps a process fluid from the pump cylinder during the extend stroke, and draws process fluid into the pump cylinder during the retract stroke. Accordingly, during each pump cycle, hydraulic system pressure peaks during the extend stroke, and declines sharply at the end of the piston stroke when the shuttle valve opens. While the shuttle valve is open, hydraulic system pressure levels off at a pressure governed by the pressure drop through the shuttle valve and fluid passage through the piston, as shown by the flat portion of the plot identified by reference number 101B at the end of the extension stroke. In the data plotted for FIG. 1, when the hydraulic piston reverses direction for a retracting stroke, the shuttle valve closes and hydraulic system pressure declines and levels off at an even lower pressure associated with the pressure drop of the hydraulic fluid flowing through an outlet from the cylinder as the hydraulic fluid is drained therefrom. At the end of the retracting stroke, hydraulic system pressure rises to again reflect the pressure drop through the open shuttle and the fluid passage through the piston, as shown by the flat portions of the plot identified by reference numbers 101A and 101A′.
The '032 application teaches a method that eliminates the need for a position or proximity sensor for the hydraulic piston, by programming an electronic controller to estimate when each piston stoke is completed as a function of hydraulic pump speed, hydraulic fluid pressure, or elapsed time. That is, because the displaced hydraulic fluid volume for each piston stroke is known, one of these variables can be used to estimate when each piston stroke is completed by calculating when the piston stroke is expected to be completed from hydraulic pump speed, hydraulic fluid pressure, or elapsed time. The '032 application teaches that the use of the shuttle valve prevents the hydraulic piston from being driven against and damaging the piston or end plates, permitting the controller to use a crude estimate of the timing for the end of the piston stroke and allowing the estimated stroke duration to include extra time for each stroke to prevent short-stroking, ensuring that the hydraulic piston completes its stroke. While this method and apparatus is effective and eliminates the need for sensors to detect when the piston has reached the end of each piston stroke, it can be improved and the hydraulic drive can be made more efficient if the extra time when the shuttle valve is open between hydraulic piston strokes can be reduced.
A number of difficulties associated with a hydraulically driven apparatus have been described above, demonstrating a need for an improved diagnostic control strategy that can be useful for addressing these and other difficulties to improve efficiency and/or operation.